1. Field of the Invention
The present invention relates to optical information record/playback devices for recording and playing back information to/from recording media by using light beams.
2. Description of the Background Art
In recent years, optical information record/playback devices for recording and playing back information such as videos, images, and audio by using light beams are widely used. Recording medium standards include, for example, compact disc (hereinafter, abbreviated as “CD”), Digital Versatile Disc (hereinafter, abbreviated as “DVD”), and Blu-ray Disc (hereafter, abbreviated as “BD”). In optical information record/playback devices, light from a semiconductor laser light source is condensed by an optical system and irradiated onto a recording medium, and reflected light from the recording medium is received by a photodetector to play back information. For recording, a light beam having power higher than that used for playback is irradiated onto a recording medium, and information is recorded by changing the structure, shape, reflectance, or the like of a portion of the recording medium on which the light has been irradiated. Here, the power of the laser irradiated on the recording medium is generally controlled through digital arithmetic processing.
Laser power control of an optical information record/playback device is described in the following by using FIG. 7.
FIG. 7 is a block diagram showing a conventional laser power control device for controlling laser power through digital arithmetic processing.
As shown in FIG. 7, the laser power control device includes an arithmetic/control circuit 22, a D/A (digital-to-analog) conversion circuit 23, resistances R1 and R2, a laser driving current source 24, a switch 32, and a semiconductor laser 1.
The D/A conversion circuit 23 includes a DATH circuit 25 for determining current required for reaching threshold power of the semiconductor laser 1, a DARD circuit 26 for determining playback-current, a DABS circuit 27 for determining bias current, and a DAPK circuit 28 for determining peak current.
The laser driving current source 24 includes a playback-current source 29 for supplying playback-current, a bias current source 30 for supplying bias current, and a peak current source 31 for supplying peak current.
The resistances R1 and R2 are resistances for adding outputs of the DARD circuit 26 and the DATH circuit 25 in the D/A conversion circuit 23.
In accordance with inputted data, the switch 32 turns ON or OFF peak current that flows in the semiconductor laser 1 from the peak current source 31.
Furthermore, the laser power control device includes a photodiode 2 for monitoring power of laser light outputted from the semiconductor laser 1, a current-voltage conversion circuit 33 for converting monitor current from the photodiode 2 into voltage, a switch 35, and an A/D (analog-to-digital) conversion circuit 36.
The monitor current outputted from the photodiode 2 is converted into voltage in the current-voltage conversion circuit 33, then passes through a variable resistance that absorbs variation in monitor efficiency, and is added to an amplifying circuit 34 of, for example, ten times.
When playback is to be carried out, a LOW level WTGT signal is supplied to the switch 35 to allow conduction between terminals a and b; and when recording is to be conducted, a HI level WTGT signal is supplied to the switch 35 to allow conduction between terminals a and c.
The A/D conversion circuit 36 conducts A/D conversion on an output signal from the terminal a of the switch 35, and outputs it to the arithmetic/control circuit 22 consisting of a microprocessor (CPU) or a digital signal processor (DSP).
The arithmetic/control circuit 22 can monitor the power of the semiconductor laser 1 based on the output from the A/D conversion circuit 36.
The operation of the laser power control device shown in FIG. 7 is divided into three modes: a playback-power control mode, a power learning mode, and a record-power control mode.
First, in the playback-power control mode, the arithmetic/control circuit 22 monitors the output (corresponds to light emission power of the semiconductor laser 1) of the A/D conversion circuit 36, and controls the outputs of the DATH circuit 25 and the DARD circuit 26 so as to maintain a playback-power P0 of the semiconductor laser 1. Here, the output values of the DATH circuit 25 and the DARD circuit 26 causing the semiconductor laser 1 to emit light with an output at the playback-power P0 are obtained when the device is assembled and calibrated, and are pre-stored in a memory inside the arithmetic/control circuit 22.
Next, in the power learning mode, the playback-power control mode is cancelled, and the outputs of the DATH circuit 25 and the DARD circuit 26 for the playback-power control mode are maintained. Then, the arithmetic/control circuit 22 obtains a slope Kr (slope=drive current/power) of the drive current around the playback-power P0, a slope Kb of the drive current around bias power Pb, and a slope Kp of the drive current around peak power Pp. The values of the obtained Kr, Kb, and Kp are stored in the memory inside the arithmetic/control circuit 22.
Then, in the record-power control mode, similar to the power learning mode, the playback-power control mode is cancelled, and the outputs of the DATH circuit 25 and the DARD circuit 26 for the playback-power control mode are maintained. By using Kb and Kp obtained in the previous power learning mode, the arithmetic/control circuit 22 obtains drive currents corresponding to desired bias power and peak power, and configures the outputs of the DABS circuit 27 and the DAPK circuit 28.
The configuration and the operation of the above described conventional laser power control device are disclosed in, for example, Japanese Laid-Open Patent Publication No. H6-338073.
FIG. 8 is a block diagram showing a laser power control circuit used in a typical playback-only device.
In the laser power control circuit shown in FIG. 8, the semiconductor laser 1 emits light upon receiving current from a current driver 10. The current driver 10 outputs current in accordance with voltage determined by a transistor 8 and a resistance 9. The light emitted from the semiconductor laser 1 is received by the photodiode 2. Photocurrent generated at the photodiode 2 is converted into voltage by a variable resistance 3, and is conducted to an inverting input of an operational amplifier 6. Voltage configured by a current source 4 and a variable resistance 5 is supplied to the other input of the operational amplifier 6, which is a non-inverting input. The output of the current driver is feedback-controlled such that these voltage values match. As a result, the output of the semiconductor laser 1 is controlled to be a desired value by the output of the current source 4 and by the values of the variable resistances 3 and 5.
Optical information record/playback devices are widely used as being incorporated in various products including personal computers, AV players, AV recorders, and the like. However, those that are currently most widely used in the world are playback-only devices mounted on AV players and home video-game machines. Since demands are extremely high to reduce prices of products that have entered a widespread-use period such as in the case with the playback-only devices, it is conceivable that demands to reduce prices of optical information record/playback devices having both functions of recording and playing back information will increase as they become widely used.
However, as described with FIG. 7, conventional laser power control devices for recording and playing back have a configuration greatly different from typical playback-only laser power control devices (FIG. 8), and require complicated arithmetic processing systems and many dedicated circuits to conduct light emission control at record-power. Therefore, if conventional laser power control devices are used, the final products, i.e., the optical information record/playback devices, become expensive, and so that it becomes difficult to attain cheap optical information record/playback devices.